There exists a wide variety of devices which utilize electrodynamic linear motors. When utilized with a reciprocating compressor or pump, for example, such an electrodynamic motor provides a driving force to a piston which provides the compression action on a working fluid, which may be a gas or a liquid.
The motor may have an armature with a coil wound about a support and positioned within a flux field provided by a permanent magnet stator means. Alternatively, the motor may have a plunger means which is positioned within the flux field. A compression piston is usually coupled to the armature and the armature held in a rest position by way of one or more main or resonance springs. When the motor is energized, a magnetic force is generated to drive the piston. The resonance spring causes the piston to oscillate back and forth to provide compression of the fluid. Arrangements which operate in a manner such as this, or similar thereto, can be found in U.S. Pat. Nos. 3,814,550 to Adams for "Motor Arrangement and Lubrication System for Oscillatory Compressor," issued June 4, 1974, 3,781,140 to Gladden for a "Synchronous Reciprocating Electrodynamic Compressor System," issued Dec. 24, 1973 and 3,325,085 to Gaus for a "Compressor," issued June 13, 1967, all of which involve a moving coil linear motor with permanent magnet excited stator field. Other compressor arrangements employing plunger-type electrodynamic motors can be found in U.S. Pat. Nos. 3,937,600 to White for a "Controlled Electrodynamic Linear Compressor" and 4,353,220 to Curwen for a "Resonant Piston Compressor Having Improved Stroke Control for Load-Following Electric Heat Pumps and the Like."
A continuing problem associated with known moving plunger electrodynamic resonant reciprocating compressors involves the total mass of the reciprocating assembly which comprises the compressor piston means and the motor plunger. Resonant operation of such compressors at the local A-C power frequency requires an appropriate amount of spring stiffness acting on the reciprocating assembly. It can be shown that the total required spring stiffness is almost directly proportional to the total mass of the reciprocating assembly, the major portion of which is determined by the mass of the motor plunger.
In one known prior art compressor system, for example, approximately 80 percent of the total mass of the reciprocating assembly was due to the motor plunger. Only 20 percent to 30 percent of the total stiffness could be supplied by the compression cylinders so that the remainder had to be supplied by gas-spring cylinders, as shown for example in U.S. Pat. Nos. 3,937,600 and 4,067,667.
While such gas-spring type resonant reciprocating compressors have been satisfactory, gas springs introduce undesirable losses and also contribute to increased manufacturing costs. It is desirable to avoid, if possible, the need for the use of gas springs by providing an electrodynamic motor having a much lighter plunger. Accordingly, there exists a need to further improve upon such electrodynamic motors for purposes of efficiency, weight and durability.